Hydraulic equipment, such as a water gate of a dam, a movable weir on a river, a hydraulic cylinder for driving an industrial machine, and a hydraulic motor, is set up nearby a driven unit. This is because actuation pressure oil from the pressure fluid source is supplied to and discharged from a driven unit of a water gate, a movable weir, a tide gate, an industrial machine, and the like via a hydraulic circuit so as to actuate the same.
The pressure fluid source is a device in which a pressure-oil tank, a hydraulic pump, an electronic device configured to supply and discharge the actuation pressure oil to/from the hydraulic equipment, or a switch valve and the like are integrated. Since such a pressure fluid source is vulnerable to rain and wind, and since the entire device may not operate due to damages to the pressure fluid source, a building is provided and the pressure fluid source is set up within the building. This causes the distance from the pressure fluid source to the hydraulic equipment to be long.
The hydraulic circuit from the pressure fluid source to the hydraulic equipment is structured by weld-joint, to a socket which is a socket-weld type pipe joint, an end portion of a steel pipe structuring the hydraulic circuit, the pipe having a straight pipe portion of a regulated length (approximately 6 meters). The bent-pipe portion is formed by bending the steel pipe by a bender and its end portion is welded to a socket. For a curvature of a radius that cannot be processed by a bender or for branches, an elbow, a Tee, or a cross each of which is a socket-weld type joint is used according to the site of piping to structure the hydraulic circuit.
As shown in FIG. 16 which illustrates the pipe and the socket before and after assembly, a socket 50 of a socket-weld type pipe joint structuring the straight pipe portion has pipe insertion holes 52 and 53 at end portions on both side of a main body 51 having a straight pipe shape. To these pipe insertion holes are inserted end portions 55a and 56a of pipes 55 and 56 structuring the hydraulic circuit, respectively.
Between the pipe insertion holes 52 and 53 is provided a protruding portion 54 which forms a reduced diameter hole 54a whose diameter corresponds to the inner diameter of the pipes 55 and 56. Between the protruding portion 54 and the pipe insertion holes 52 and 53 are contact surfaces 52a and 53a. With the socket 50, the pipe 55, and the pipe 56, a straight pipe portion in the hydraulic circuit is structured as follows. Namely, taking the pipe 55 for example, the pipe 55 is inserted into the pipe insertion hole 52 until the end portion 55a of the pipe 55 abuts the contact surfaces 52a as shown on the right side of FIG. 16, and the outer circumference of the pipe 55 and the socket 50 are welded at a weld portion 58, as shown on the left side of FIG. 16.
As shown in FIG. 17 which illustrates the pipe and elbow before and after assembly, an elbow 60 of the socket-weld type pipe joint structuring the bent-pipe portion has, at the both ends of the main body 61 in a shape of a bent pipe, pipe insertion holes 62 and 63. To these pipe insertion holes 62 and 63 are inserted end portions 65a and 66a of pipes 65 and 66 structuring the hydraulic circuit, respectively. Between the pipe insertion holes 62 and 63 are provided a protruding portion 64 which forms a reduced diameter hole 64a whose diameter corresponds to the inner diameter of the pipes 65 and 66. Between the protruding portion 64 and the pipe insertion holes 62 and 63 are contact surfaces 62a and 63a. 
With this elbow 60, a bent-pipe portion in the hydraulic circuit is structured as follows. Namely, an end portion 65a of the pipe 65 is inserted until its leading end portion abuts the contact surfaces 62a, and then the outer circumference of the pipe 65 and the main body 61 in the shape of a bent pipe are welded to each other at a weld portion (similar to weld portion 68). Meanwhile, an end portion 66a of the pipe 66 is inserted until its leading end portion abuts the contact surfaces 63a, and then the outer circumference of the pipe 66 and the main body 61 in the shape of a bent pipe are welded to each other at a weld portion 68.
As hereinabove mentioned, the socket 50 and the elbow 60 for structuring a straight pipe portion and a bent-pipe portion of a piping structure by, for example, inserting an end portion (end portions 55a, 56a, 65a, and 66a) of a pipe (pipe 55, 56, 65, and 66) into a pipe insertion hole (pipe insertion holes 52, 53, 62, and 63), and welding the outer circumference of the pipe to the socket 50 or the elbow 60.
In the piping structure, for example, the diameter of the pipe insertion hole of the pipe joint, to which an end portion of a pipe is inserted, is slightly greater (by approximately 0.5 mm in the nominal diameter 20A) than the outer diameter of the pipe for the sake of workability. This creates a gap A between the outer circumference of the pipe and the pipe insertion hole. Therefore, the surface of the pipe is oxidized by the heat from welding of the pipe inserted and the pipe joint, and this oxidized surface remains as an oxidized layer on the surface of the pipe, i.e., in the gap A.
Such an oxidized layer is thin and sharp, and is extremely hard since it is an oxide of iron. During the operation of the hydraulic device, the oxide may depart the gap A and become a foreign matter, which floats within the pressure oil and circulates the hydraulic line. The foreign matter may enter the hydraulic equipment, the control valves, and the hydraulic equipment, and may damage the sliding surfaces, consequently leading to a malfunction.
Further, the hydraulic oil is subject to a high pressure, for the purpose of driving the hydraulic equipment. Further, through the hydraulic equipment, water or air may enter the hydraulic line, and actuation of the hydraulic pump, valves, and cylinders may cause entrance of various foreign matters such as metal powder and residues caused by damages to sealing.
The various foreign matters such as the air and water, which are mixed into the hydraulic oil, activate the hydraulic oil with an increase in the temperature caused by operation of the hydraulic equipment, and becomes a contaminant such as resin and sludge which deteriorates the hydraulic oil and the function of the hydraulic equipment.
As shown in FIG. 18 (a) and FIG. 18 (b), PTL 1 discloses a structure in which a weld width h is arranged between an end portion 76a of a pipe 76 inserted into a pipe insertion hole 73 of a pipe joint 70 and a contact surface 73a of the pipe insertion hole 73, and a gap A between the pipe insertion hole 73 and the outer circumference of the pipe 76 is sealed by welding at the weld portion 79. Further, FIG. 19 shows a structure in which a weld width h is arranged between an end portion 86a of a pipe 86 inserted into a pipe insertion hole 83 of a pipe joint 80 and a contact surface 83a of the pipe insertion hole 83, and a gap between the outer circumference of the pipe 86 and the pipe insertion hole 83 is sealed by welding at the weld portion 89.
The above connection structure of the pipe joint 70 and the pipe 76 indeed seals the foreign matters remaining in the gap A. However, it is the end portion 76a of the pipe 76 and the contact surfaces 73a which are welded at the weld portion 79 within the weld width h, there will be a gap h1 between the end portion 76a and the contact surfaces 73a. It should be noted that a gap h1 is also formed in the connection structure of the pipe joint 80 and the pipe 86, as shown in FIG. 19.
The piping structure 70c of the straight pipe shown in FIG. 19 is identical to the piping structure having the pipe joint 70, and the piping structure having the pipe joint 80 is fastened by hexagon socket head cap bolts 82.
It should be noted that the pipe joint 70 has four coupling holes 71a provided along a circle concentric with the connection hole 74. Further, the pipe joint 80 to be screw-fastened to the pipe joint 70 has coupling holes 81a in positions corresponding to the four coupling holes 71a, respectively. Further, the coupling holes 71a and the coupling holes 81a have a threaded hole structure and a threaded through-hole structure; i.e., where one of them has a threaded hole structure, the corresponding counterpart has the threaded through-hole structure. These structures are stipulated in JISB2291.
In the case of the pipe joint 70 and the pipe joint 80, the coupling holes 71a on the pipe joint 70 are through holes, while the coupling holes 81a on the pipe joint 80 is structured as a female screw. Further, an O-ring 72 for sealing the pipe joint 70 and the pipe joint 80 is arranged on the connection surface 77 of the pipe joint 70. The other structures are substantially the same, and therefore descriptions are provided but details are omitted as needed.
In the piping structure 70C of a straight pipe structured by fastening the pipe joint 70 and the pipe joint 80 by using the four hexagon socket head cap bolts 82, there will gaps h1 in number corresponding to the number of pipe joints used in the piping structure structuring the straight pipe.